Estimating molybdenum content of scheelite or calcium tungstate by visual color of its fluorescence



p 4- R. s. CANNON, JR, ETAL 2,346,661

ESTIMATING MOLYBDENUM CONTENT OF SCHEELITE 0R CALCIUM TUNGS'I'ATE BY VISUAL COLOR OF ITS FLUORESCENCE Filed Oct. 12, 1942 A WE/G HT PERCENT Ma.

Meu/vm J. MURATA INVENTOR R LPH SMYSEE. CANNON, J/a,

' ATTORNEY Patented Apr. 18, 1944 ESTIIVIATING MOLYBDENUM CONTENT OF SCHEELITE OR CALCIUM TUNGSTATE BY VISUAL COLOR OF ITS FLUORESCENCE Ralph S. Cannon, Jr., Falls Church, Va., and Kiguma J. Murata, Washington, D. C.

Application October 12, 1942, Serial No. 461,830

(Granted under the act of March 3, 1883, as,

amended April 30, 1928; 370 0. G. 757) 15 Claims.

The invention described herein may be made and used by and for the Government of the United States for governmental purposes without the payment to us of any royalty therefor.

This invention relates to methods and means for determining the molybdenum content of scheelites, i. e., members of the scheelite-powellite series, and the like, and aims generally to improve the same.

It aims particularly but not exclusively to provide a method and means for estimating the molybdenum content of scheelite which may be used upon deposits as they are discovered, with results suificiently accurate to permit evaluation of the ore.

In the past while scheelite (a common domesti ore of tungsten) has been located by its fluorescence in ultra-violet light, the presence of molybdenum in the scheelite and its amount, has been detected only by testing in assay laboratories.

Commercial scheelite-concentrates containing more than four-tenths of one percent molybdenum are subject to a price penalty because of requirements of the metallurgical arts, and while the intensive search for tungsten in the United States begun by the Geological Survey, U. S, Department of the Interior, in 1938, has been successful, considerable of the scheelite encountered has an undesirable molybdenum content. Indeed most of the scheelite in the contact-metamorphic ite series, and also for synthesized specimens of the isomorphous calcium tungstate-calcium molybdate series. Our research has shown, as could not be predicted, that the fluorescence color given by these materials varies systematically with the proportions of tungsten and molybdenum there-. in, and our invention applies this new discovery to provide a method and means of estimating the molybdenum content of such minerals, In thecourse of our research we prepared andtested 2.

series of synthetic compounds ranging in composition from pure calcium tungstate to pure calcium molybdate according to the method described by L. Michel, Bull. Soc. Francaise Minerol.

, vol. II, page 142, year 1879, and vol. XVII, page 612, year 1894', and also checked the composition indications obtained by our herein described method against complete chemical analyses of selected samples of scheelites and powellites from the Seven Devils and other western districts, with, surprisingly, entirely consistent results.

With the type of ultra-violet lamp commonly used in prospecting for scheelite, which produces light, covering the wavelength range of 2500 to 4000 Angstrom units and largely concentratedbetween 2500 and 2600 Angstrom units, we have found that the fluorescence colors of the finely powdered synthetic preparations, and of pow-, dered natural counterparts thereof, are bright blue for pure calcium tungstate, paler blue forcompounds containing traces of molybdenum, and neutral white when the molydenum content is about 0.5%. As the molybdenum content rises above 0.5% the fluorescence color becomesincreasingly yellow. It is strongly yellow for a compound containing 4.8% molybdenum; and strangely enough it remains at substantially the same shade of yellow for any compound containing a higher percentage of molybdenum (any change being so gradual as to be virtually unnoticeable).

Our researches have further shown that when scheelite is exposed to an ultra-violet lamp having a range of wave lengths comprising only those longer than about 3000 Angstrom units, the blue fluorescence of the pure tungsten mineral is not obtained; if there is present a small fraction of a percent of the molybdate a feeble fiuores cence of uncertain color is obtained; thus the use of wavelengths shorter than 3000 Angstromunits is preferred, and, wherethe observations are made in air, wavelengths not within the air absorption range (2200 Angstroms and shorter in g the ultraviolet range). It is possible to use ultraviolet light containing only wavelengths of 3000 Angstroms and longer, but in this instance, while the samples containing about 0.05% molybdenum and up fiuoresce they do so with no change ofcolor enabling discrimination between them, so

they can only be distinguished as a groupfrom;

the pure tungstate mineral which becomes identified by the fact that it does not fiuoresce at all. Thus, unless wave lengths of less than 3000 Angstrom units are present, the full color discrimination provided by the invention is not realized.

While we have determined that the yellow and blue fluorescence colors comprise wide bands of spectrum color having maximum intensities in the yellow and blue regions, respectively, we have not as yet been able fully to account for the fact that the color transition passes through a pure white in its shift from the blue to the yellow fluorescence, or to explain the strange and most useful fact that this fluorescence color die-- velopment so exactly parallels the economic value of the scheelite to the tungsten industry as to afford practically a go-no go check test by color alone, as well as a quantitative test by shade of color,

The gono go check test results from the fact that concentrates not penalized for molybdenum can be made from any scheelite that fluoresces distinctly blue; while those that fluoresce white are border-line cases containing roughly from 0.35 to 1.0% molybdenum; and those that fluoresce distinctly yellow contain more than 1.0 molybdenum. Thus a quick evaluation of molybdenum content may be made in the field by employing our discovery, without ever resorting to shade measuring; while more exact quantitative determination of the molybdenum content of any scheelite containing less than 4% molybdenum can also quickly be made in the field by suitable measurement of the fluorescence shade of the powdered material in accordance with our invention. 1

The new method and means of our invention in this latter respect provide for determining quantitatively the proportion of molybdenumin the tungsten mineral, preferably picked out of the crude ore-up to a practical limit of about 4%, and provide for identifying materials containing percentages of molybdenum between 4.5 and 4.8% (the percentage in pure powellite, which shows the yellowest possible fluorescence hue), as lying in the latter range.

To illustrate the close accuracy of quantitative results obtained in the practice of our invention on scheelite: a crystal, picked out of a crude ore sample obtained from the Nightingale mine in Nevada, and ground to a powder, was estimated from its fluorescence to contain considerably less than 0.050% molybdenum; chemical analysis showed 0.015%. Another scheelite, from the Tungstar mine in California, was estimated to contain 0.57% molybdenum on the basis of its fluorescence color; chemical analysis showed 0.51%. A third scheelite, from the Seven Devils district, was found to contain 1.20% molybdenum by chemical analyses confirming a previous estimate of 1.18% from'the fluorescence color. And in all these instances the fluorescence color was estimated simply by matching against a card of the type hereinafter described, without any spectrographic analysis at all.

From What has been said before, it will be perceived that the present invention provides a method of rapidly estimating, either on a go-no go basis, or with close quantitative accuracy within the critical region, the molybdenum content of scheelite by the employment of ultra-violet light lying within the range of 2000 to 4000 Angstrom units and containing wave lengths less than about 3000 Angstrom units; that the determination of molybdenum content, as indicated by fluorescence color, may be estimated in any suitable manner with or without special apparatus without departure from the invention; and that the invention in its preferred form provides also a simple and convenient means consisting of a plurality of standard fluorescible samples, preferably finely powdered and mounted in a unitary structure, for more exact quantitative determination by fluorescence color matching.

While the structure comprising the plurality of standard fluorescence color standards may :be embodied in many forms, a simple and presently preferred form is shown in the accompanying drawing in which:

The single figure is a plan View of a fluorescible sample card constituting suitable fluorescence matching apparatus, with sample carrying portions shaded to indicate the fluorescence colors developed by such samples dependent on their molybdenum content as designated by the indicia shown adjacent thereto.

Referring to the embodiment illustrated, the several powdered samples of calcium tungstatecalcium molybdate are mounted in a unitary structure, herein the card A, in any suitable manner. The card A may be formed of a single lamina, punched with apertures B and C, of which the apertures B serve as apertures through which to View the unknown specimen, while the apertures C serve to receive the respective standard fluorescence samples indicated. These samples may be mounted in the apertures C in any suitable manner, as by affixing sections of non-drying cellulosic or other adhesive tape across the backs of apertures C and embedding the powdered samples on the adhesive faces exposed through the apertures, or by covering the back of the card A with a second lamina, having apertures B only, and carrying, in position to be exhibited through the apertures C, the respective powdered standardized samples afiixed in an at least substantially non-fluorescent binder of ma-- terial transmissive to ultraviolet light and transmissive of the color of the fluorescence light of the sample.

By either of these arrangements the recessing of the samples below the surface of the card constitutes a means for protecting the samples against being rubbed oif. To the same end, if desired, the finely powdered samples may be covered with a thin sheet of material transmissive to ultraviolet light and to the fluorescence light of the sample, and if the material tends to slightly alter the shade of the fluorescence color, a cover of the same material may be provided to be placed over the unknown powdered specimen during fluorescence matching.

The apertures B and C, in the form shown, constitute a preferred arrangement providing for close juxtaposition to the known samples of the unknown specimen of scheelite being investigated;

As is preferred, the samples C, which in the illustrated example have the compositions and which the card A may be moved until the closest fluorcescent color match is obtained.

Table I.Data for card shown in drawin C t Pei;j Pei; Per cent Pczr e)nt lllTEl] cen 0611 W W Color of trade (wt.) iwt.) calcium calcium value of of molybtungsfluorescence Mo W; date tate .00 80. 6 0.0 100.0 Blue. No penalty 80. 5 0. 1 99. 9 Pale blue.

19 80.2 0. 4 99. 6 Paler blue. 33 80.0 0.7 99. 3 whitish blue. .48 79. 8 l. 0 99. 0 Bluish white. Border line 72 79. 4 1. 5 98. 5 White.

' .96 79.0 2.0 98.0 Yellowish white.

1.4 78.2 3.0 97.0 Whitish yellow. Price penal- '2. 4 76. 6 5.0 95. 0 Pale yellow. izod 13. 4 74.9 7.0 93. 0 Less pale yellow. 4. 8 72. 4 10. 0 90. 0 Yellow. 48.0 0.0 4 100.0 0.0 Do.

Inthose cases in which it is desirable to mix the finely powdered samples in a body of binder,

we have found that cellulose nitratecement' propriate portion of the card or the like carrier. Where two laminae are used, the respective sample slurries may be painted onto the underlying lamina in areas somewhat larger than the apertures C, before superposition of the surface lamina thereon, thus adding to the neatness of the structure. While this dilute cellulose nitrate binder seems entirely satisfactory for samples containing a few hundredths of a percent of molybdenum, the pure calcium tungstate (0.00 in the drawing) requires quite short ultraviolet rays to excite its fluorescence, and this binder does not transmit those waves as well as longer ones. For this reason, it is preferred, when using this binder for the other samples, to embed the pure calcium tungstate sample on an adhesive surface, such, for example as Scotch tape.

As the samples are to be viewed in ultraviolet light it is desirable, though not essential, that the card have a substantially non-fluorescent surface. and one which preferably is a dark color to reduce reflection of visible light to a minimum. Such provisions render easier the exact matching of the fluorescence colors of the standard and unknown samples. A black, nonfluorescent, surface is preferred.

As will be apparent from the foregoing description, the invention is not limited to the particular embodiment described to illustrate its several novel features.

We claim as our invention:

1. A method of rapidly estimating the'molybdenum content of scheelite which consists in ex-' posing a sample of the scheelite to a range of.

ultra-violet light comprising wave lengths lying in the range of 2000 to 3000 Angstrom units, and determining the extent to which its fluorescence color departs from blue through white toward yellow, which reflects and thus indicates the molybdenum content of the scheelite.

2. A method according to claim 1 in which the determination of the molybdenum content is made by comparing the fluorescence color of the unknown sample with the graded fluorescence of a series of graded samples of known composition selected from the group consisting of samples, of natural and artificial compoundslof the. isomorphcus calcium tungstate-calcium molybdate series. covering said color range and exposed tovsimilar ultra-violet light.

.:3.'A method according to claim 1 in which the determination of the molybdenum content is made byFmatching the fluorescence color of the unknown sample to the graded fluorescence of a considerable number of graded samples of known different; composition selected from the group consisting of powdered samples of natural and artificial compounds of the isomorphous calcium tungstate-calcium molybdate series covering said color range and exposed to similar ultraviolet light.

.4:.A method according to claim 1 in whichthe' determination of the molybdenum content is made by matching the fluorescence color of the unknown sample with the graded fluormcence of a series of graded samples of known composition selected from the group consisting of powdered samples of natural and artificial compounds of the isomorphcus calcium tungstatecalcium modybdate series including samples having less than 5% molybdenum content and exposed to similar ultra-violet light.

5-. A method of rapidly estimating the molybdenum content of powered scheelite which consists in selecting powdered specimens of determined composition from the group consisting of (a) natural minerals of the isomorphous scheelite-powellite series and (b) synthesized specimens of the isomorphcus calcium tungstate-calcium molybdate series-which series grade continuously in fluorescence color under ultra-violet light from blue through white to yellow in the composition range from pure calcium tungstate to a compound of calcium tungstate and calcium molybdate in the molecular proportion of 6 to 1 (i. e. a calcium tungstate compound containing approximately 10.0% by weight of calcium molybdate amounting to 4.8% by weight of molybdenum) and which are of substantially the same hue of yellow for the compounds containing molybdenum in greater proportions to and includin pure calcium molybdate; arranging said specimens in sequential arrangement of increasing known molybdenum content; exposing the group of sequentially arranged specimens to ultra-violet light having a wavelength range of approximately 2500 to 4000 Angstrom units; simultaneously exposing the powdered unknown sheelite to similar ultra-violet light which may be from the same source; and comparing visually the color of fluorescence of the unknown sample with the fluorescence colors of the known specimens, until the nearest match of fluorescence colors is obtained indicating the proportion of molybdenum in the unknown sample.

6. Apparatus for rapidly estimating the molybdenum content of sheelite with the aid of ultraviolet light consisting of a series of graded samples of known composition selected from the group consisting of natural and artificial compounds of the isomorphcus calcium tungstatecalcium molybdate series, said series of samples including samples having less than 5% molybdenum content and varying in fluorescence color under ultra-violet light directly with their molybdenum content from blue through white toward yellow.

7. Apparatus according to claim 6 in which the graded samples of the series are finely powdered.

8. Apparatus according to claim 6 in which graded samples of the series are mounted in a unitary structure and arranged for close juxtaposition to the individual samples therein of a scheelite sample of unknown composition.

9. Apparatus according to claim 6 in which the graded samples of the series are mounted in progressive order of the increase of molybdenum content thereof in a unitary structure having apertures between the respective samples to facilitate viewing of the sample of unknown composition in interposition to adjacent ones of the graded samples.

10. Apparatus according to claim 6 in which each sample is finely powdered and mounted on the surface of an adhesive carrier.

11. Apparatus according to claim 6 in which each sample is finely powdered and carried by an at least substantially non-fluorescent binder of a material transmissive to ultra-violet light and transmissive of the color of the fluorescence light of the sample.

12. Apparatus according to claim 6 in which each sample is finely powdered and covered by a thin sheet of material transmissive to ultra-violet light and to the fluorescence light of the sample, and in which the apparatus further includes a cover for the unknown specimen consisting of a thin sheet of the same transmissive material.

13; Apparatus according to claim 6 comprising a perforated card having the individual samples of said series of samples mounted in sequential arrangement in alternate ones of its perforations, with the interpositioned perforations open to 'provide for observation of the fluorescence of a sample of unknown composition positioned thereat.

14. Apparatus according to claim 6 comprising a perforated card having a substantially nonfiuorescent non-reflective surface and having the individual samples of said series of samples mounted in sequential arrangement in alternate ones of its perforations, with the interpositioned perforations open to provide for observation of the fluorescence of a sample of unknown composition positioned thereat.

15. Apparatus according to claim 6 comprising a perforated card having a substantially nonfluorescent black surface and having the individual samples of said series of samples mounted in sequential arrangement in alternate ones of its perforations, with the interpositioned perforations open to provide for observation of the fluorescence of a sample of unknown composition p0-- sitioned thereat.

' RALPH S. CANNON, JR.-

KIGUMA J. MURATA.

CERTIFI GATE OF CO RRECTI ON Patent No. 2, |.6,66l. April 8, 9%

RALPH S. CANNON, JR. ET AL.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2, first column, line 11.5, for "l;..8%" read l 8%-; page 5, first column, line 29, strike out "the" before "like"; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 25th day of July, A. D. 19th.

Leslie Frazer (Seal) Acting Commissioner of Patents. 

